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      mongodb笔记(六) 聚合
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			<h2 class="toc-title"><span>Contents</span></h2>
		
			<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#count"><span class="toc-number">1.</span> <span class="toc-text">count</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#distinct"><span class="toc-number">2.</span> <span class="toc-text">distinct</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#group"><span class="toc-number">3.</span> <span class="toc-text">group</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#使用完成器"><span class="toc-number">3.1.</span> <span class="toc-text">使用完成器</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#将函数作为键使用"><span class="toc-number">3.2.</span> <span class="toc-text">将函数作为键使用</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#MapReduce"><span class="toc-number">4.</span> <span class="toc-text">MapReduce</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#找出集合中的所有键"><span class="toc-number">4.1.</span> <span class="toc-text">找出集合中的所有键</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#网页分类"><span class="toc-number">4.2.</span> <span class="toc-text">网页分类</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#MongoDB和MapReduce"><span class="toc-number">4.3.</span> <span class="toc-text">MongoDB和MapReduce</span></a></li></ol></li></ol>
		
		</div>
		
        <h2 id="count">count</h2><p>返回集合中的文档数量<br><figure class="highlight stata"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">db</span>.foo.<span class="keyword">count</span>()</span><br><span class="line"><span class="keyword">db</span>.foo.<span class="keyword">count</span>(&#123;<span class="string">"x"</span>:1&#125;) <span class="comment">//也可以传递查询，但会使count变慢</span></span><br></pre></td></tr></table></figure></p>
<a id="more"></a>
<h2 id="distinct">distinct</h2><p>找出给定键的所有不同的值，使用时必须指定集合和键</p>
<p>假设现在有如下文档：<br><figure class="highlight json"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">&#123;"<span class="attribute">name</span>":<span class="value"><span class="string">"ada"</span></span>,"<span class="attribute">age</span>":<span class="value"><span class="number">20</span></span>&#125;</span><br><span class="line">&#123;"<span class="attribute">name</span>":<span class="value"><span class="string">"pred"</span></span>,"<span class="attribute">age</span>":<span class="value"><span class="number">35</span></span>&#125;</span><br><span class="line">&#123;"<span class="attribute">name</span>":<span class="value"><span class="string">"susan"</span></span>,"<span class="attribute">age</span>":<span class="value"><span class="number">60</span></span>&#125;</span><br><span class="line">&#123;"<span class="attribute">name</span>":<span class="value"><span class="string">"andy"</span></span>,"<span class="attribute">age</span>":<span class="value"><span class="number">35</span></span>&#125;</span><br></pre></td></tr></table></figure></p>
<p>吐过对”age”键使用distinct，会获得不同的年龄<br><figure class="highlight stylus"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">db.<span class="function"><span class="title">runCommand</span><span class="params">(&#123;<span class="string">"distinct"</span>:<span class="string">"people"</span>,<span class="string">"key"</span>,<span class="string">"name"</span>&#125;)</span></span></span><br></pre></td></tr></table></figure></p>
<h2 id="group">group</h2><p>group做的聚合稍微复杂一些。先选定分组所依据的键，而后MongoDB就会将集合依据选定的键值的不同分成若干组。</p>
<figure class="highlight json"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">&#123;"<span class="attribute">day</span>" : <span class="value"><span class="string">"2010/10/03"</span></span>, "<span class="attribute">time</span>" : <span class="value"><span class="string">"10/3/2010 03:57:01 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.23</span></span>&#125;</span><br><span class="line">&#123;"<span class="attribute">day</span>" : <span class="value"><span class="string">"2010/10/04"</span></span>, "<span class="attribute">time</span>" : <span class="value"><span class="string">"10/4/2010 11:28:39 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.27</span></span>&#125;</span><br><span class="line">&#123;"<span class="attribute">day</span>" : <span class="value"><span class="string">"2010/10/03"</span></span>, "<span class="attribute">time</span>" : <span class="value"><span class="string">"10/3/2010 05:00:23 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.10</span></span>&#125;</span><br><span class="line">&#123;"<span class="attribute">day</span>" : <span class="value"><span class="string">"2010/10/06"</span></span>, "<span class="attribute">time</span>" : <span class="value"><span class="string">"10/6/2010 05:27:58 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.30</span></span>&#125;</span><br><span class="line">&#123;"<span class="attribute">day</span>" : <span class="value"><span class="string">"2010/10/04"</span></span>, "<span class="attribute">time</span>" : <span class="value"><span class="string">"10/4/2010 08:34:50 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.01</span></span>&#125;</span><br></pre></td></tr></table></figure>
<p>想获得的每天最后的价格列表</p>
<figure class="highlight json"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">[</span><br><span class="line">&#123;"<span class="attribute">time</span>" : <span class="value"><span class="string">"10/3/2010 05:00:23 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.10</span></span>&#125;,</span><br><span class="line">&#123;"<span class="attribute">time</span>" : <span class="value"><span class="string">"10/4/2010 11:28:39 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.27</span></span>&#125;,</span><br><span class="line">&#123;"<span class="attribute">time</span>" : <span class="value"><span class="string">"10/6/2010 05:27:58 GMT-400"</span></span>, "<span class="attribute">price</span>" : <span class="value"><span class="number">4.30</span></span>&#125;</span><br><span class="line">]</span><br></pre></td></tr></table></figure>
<p>先把集合按照天分组，然后在每一组里取包含最新时间戳的文档。<br><figure class="highlight ceylon"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">db.runCommand(&#123;<span class="string">"group"</span> : &#123;</span><br><span class="line"><span class="string">"ns"</span> : <span class="string">"stocks"</span>,</span><br><span class="line"><span class="string">"key"</span> : <span class="string">"day"</span>,</span><br><span class="line"><span class="string">"initial"</span> : &#123;<span class="string">"time"</span> : <span class="number">0</span>&#125;,</span><br><span class="line"><span class="string">"$reduce"</span> : <span class="keyword">function</span>(<span class="annotation">doc</span>, prev) &#123;</span><br><span class="line"> <span class="keyword">if</span> (<span class="annotation">doc</span>.time &gt; prev.time) &#123;</span><br><span class="line"> prev.price = <span class="annotation">doc</span>.price;</span><br><span class="line"> prev.time = <span class="annotation">doc</span>.time;</span><br><span class="line"> &#125;</span><br><span class="line">&#125;&#125;&#125;)</span><br></pre></td></tr></table></figure></p>
<p>“ns” : “stocks”<br>指定要进行分组的集合<br>“key” : “day”<br>指定文档分组依据的键。day值相同的文档被划分到一组<br>“initial” : {“time” : 0}<br>每一组reduce函数调用的初始时间，会作为初始文档传递给后续过程。每一组的所有成员都会使用这个累加器，所以改变会保留<br>“$reduce” : function(doc, prev) { … }<br>每个文档都对应一次这个调用。有两个参数，当前文档和累加器文档（本组当前的结果）。每一组都有独立的累加器。</p>
<figure class="highlight perl"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">db.runCommand(&#123;<span class="string">"group"</span> : &#123;</span><br><span class="line"><span class="string">"ns"</span> : <span class="string">"stocks"</span>,</span><br><span class="line"><span class="string">"key"</span> : <span class="string">"day"</span>,</span><br><span class="line"><span class="string">"initial"</span> : &#123;<span class="string">"time"</span> : <span class="number">0</span>&#125;,</span><br><span class="line"><span class="string">"<span class="variable">$reduce</span>"</span> : function(doc, prev) &#123;</span><br><span class="line"> <span class="keyword">if</span> (doc.<span class="keyword">time</span> &gt; prev.<span class="keyword">time</span>) &#123;</span><br><span class="line"> prev.price = doc.price;</span><br><span class="line"> prev.<span class="keyword">time</span> = doc.<span class="keyword">time</span>;</span><br><span class="line"> &#125;&#125;,</span><br><span class="line"><span class="string">"condition"</span> : &#123;<span class="string">"day"</span> : &#123;<span class="string">"<span class="variable">$gt</span>"</span> : <span class="string">"2010/09/30"</span>&#125;&#125;</span><br><span class="line">&#125;&#125;)</span><br></pre></td></tr></table></figure>
<p>添加”condition” 可以获得最近30天的价格。”condition”和”cond”及”q”键完全一样。</p>
<p>最后返回由30个文档返回的数组，每个组一个文档。魅族还有分组依据的键以及这组最终的prev值。如果有的文档没有依据的键，就都会被分到一组，相应的部分就会使用”day:null”。在”condition”中加入<code>&quot;day&quot;:{&quot;$exists&quot;:true}</code>可以去掉。</p>
<h3 id="使用完成器">使用完成器</h3><p>完成器用于精简从数据库传到用户的数据。<br><figure class="highlight actionscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line">db.runCommand(&#123;<span class="string">"group"</span> : &#123;</span><br><span class="line"><span class="string">"ns"</span> : <span class="string">"posts"</span>,</span><br><span class="line"><span class="string">"key"</span> : &#123;<span class="string">"day"</span> : <span class="literal">true</span>&#125;,</span><br><span class="line"><span class="string">"initial"</span> : &#123;<span class="string">"tags"</span> : &#123;&#125;&#125;,</span><br><span class="line"><span class="string">"$reduce"</span> : <span class="function"><span class="keyword">function</span><span class="params">(doc, prev)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">for</span> (i <span class="keyword">in</span> doc.tags) &#123;</span><br><span class="line"> <span class="keyword">if</span> (doc.tags[i] <span class="keyword">in</span> prev.tags) &#123;</span><br><span class="line"> prev.tags[doc.tags[i]]++;</span><br><span class="line"> &#125; <span class="keyword">else</span> &#123;</span><br><span class="line"> prev.tags[doc.tags[i]] = <span class="number">1</span>;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;,</span><br><span class="line"><span class="string">"finalize"</span> : <span class="function"><span class="keyword">function</span><span class="params">(prev)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">var</span> mostPopular = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">for</span> (i <span class="keyword">in</span> prev.tags) &#123;</span><br><span class="line"> <span class="keyword">if</span> (prev.tags[i] &gt; mostPopular) &#123;</span><br><span class="line"> prev.tag = i;</span><br><span class="line"> mostPopular = prev.tags[i];</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="keyword">delete</span> prev.tags</span><br><span class="line">&#125;&#125;&#125;)</span><br></pre></td></tr></table></figure></p>
<p>finalize附带一个函数，在每组结果传递到客户端之前被调用一次。<br>如果没有 finalize 键，将会返回每天各个标签的个数。而不是每天最多的标签。</p>
<h3 id="将函数作为键使用">将函数作为键使用</h3><p>如果按类别分组要消除大小写的影响，就要定义一个函数来确定文档所依据的键。<br><figure class="highlight actionscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">db.posts.group(&#123;<span class="string">"ns"</span> : <span class="string">"posts"</span>,</span><br><span class="line"><span class="string">"$keyf"</span> : <span class="function"><span class="keyword">function</span><span class="params">(x)</span> </span>&#123; <span class="keyword">return</span> x.category.toLowerCase(); &#125;,</span><br><span class="line"><span class="string">"initializer"</span> : ... &#125;)</span><br></pre></td></tr></table></figure></p>
<p>使用$keyf就能依据各种复杂的条件进行分组了</p>
<h2 id="MapReduce">MapReduce</h2><p>MapReduce 是一个可以轻松并行化到多个服务器的聚合方法。它会拆分问题，再将各个部分发送到不同的机器上，让每台机器都完成一部分。当所有机器都完成的时候，再把结果汇集起来形成最终完整的结果。</p>
<p>分为几个步骤。最开始是映射(map)，将操作映射到集合中的每个文档。这个操作要么“无作为”，要么“产生一些键和x个值”。然后就是中间环节，称作洗牌(shuffle)，按照键分组，并将产生的键值组成列表放到对应的键中。化简(reduce)则把列表中的值化简成一个单值。这个值被返回，然后接着进行洗牌，知道每个键的列表只有一个值为止，这个值也就是最后的结果。</p>
<h3 id="找出集合中的所有键">找出集合中的所有键</h3><p>map函数使用函数emit返回处理的值。emit会给MapReduce一个键和一个值。这里用emit将文章某个键的计数(count)返回。<br>这样就有了许许多多{count:1}文档，每个都与集合中的一个键相关。这种由一个或多个{count:1}文档组成的数组，会传递给reduce函数。reduce函数有两个参数，一个是key，也就是emit返回的第一个值，还有另外一个数组，由一个或者多个对应于键{count:1}文档组成<br><figure class="highlight actionscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line">map = <span class="function"><span class="keyword">function</span><span class="params">()</span> </span>&#123;</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">var</span> key <span class="keyword">in</span> <span class="keyword">this</span>) &#123;</span><br><span class="line">emit(key, &#123;count : <span class="number">1</span>&#125;);</span><br><span class="line">&#125;&#125;;</span><br><span class="line">reduce = <span class="function"><span class="keyword">function</span><span class="params">(key, emits)</span> </span>&#123;</span><br><span class="line">total = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i <span class="keyword">in</span> emits) &#123;</span><br><span class="line"> total += emits[i].count;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">return</span> &#123;<span class="string">"count"</span> : total&#125;;</span><br><span class="line">&#125;</span><br><span class="line">mr = db.runCommand(&#123;<span class="string">"mapreduce"</span> : <span class="string">"foo"</span>, <span class="string">"map"</span> : map, <span class="string">"reduce"</span> : reduce&#125;)</span><br><span class="line">&#123;</span><br><span class="line"><span class="string">"result"</span> : <span class="string">"tmp.mr.mapreduce_1266787811_1"</span>,</span><br><span class="line"><span class="string">"timeMillis"</span> : <span class="number">12</span>,</span><br><span class="line"><span class="string">"counts"</span> : &#123;</span><br><span class="line"><span class="string">"input"</span> : <span class="number">6</span></span><br><span class="line"><span class="string">"emit"</span> : <span class="number">14</span></span><br><span class="line"><span class="string">"output"</span> : <span class="number">5</span></span><br><span class="line">&#125;,</span><br><span class="line"><span class="string">"ok"</span> : <span class="literal">true</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<ul>
<li><p>“result” : “tmp.mr.mapreduce_1266787811_1”</p>
<p>  存放MapReduce结果的集合名，是个临时集合，MapReduce连接关闭后自动就被删除了。</p>
</li>
<li><p>“timeMillis” : 12</p>
<p>  操作花费时间，单位是毫秒</p>
</li>
<li><p>“counts” : { … }</p>
<p>  这个内嵌文档包含3个键</p>
</li>
<li><p>“input” : 6</p>
<p>  发送到map函数的文档个数</p>
</li>
<li><p>“emit” : 14</p>
<p>  在map函数中emit被调用的次数</p>
</li>
<li><p>“output” : 5</p>
<p>  结果集合中创建的文档数量</p>
</li>
</ul>
<p>reduce一定要能被反复调用，不论是映射环节还是前一个简化环节。所以reduce返回的文档必须能作为reduce的第二个参数的一个元素。<br><figure class="highlight applescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">&gt; r1 = reduce(<span class="string">"x"</span>, [&#123;<span class="command">count</span> : <span class="number">1</span>, <span class="property">id</span> : <span class="number">1</span>&#125;, &#123;<span class="command">count</span> : <span class="number">1</span>, <span class="property">id</span> : <span class="number">2</span>&#125;])</span><br><span class="line">&#123;<span class="command">count</span> : <span class="number">2</span>&#125;</span><br><span class="line">&gt; r2 = reduce(<span class="string">"x"</span>, [&#123;<span class="command">count</span> : <span class="number">1</span>, <span class="property">id</span> : <span class="number">3</span>&#125;])</span><br><span class="line">&#123;<span class="command">count</span> : <span class="number">1</span>&#125;</span><br><span class="line">&gt; reduce(<span class="string">"x"</span>, [r1, r2])</span><br><span class="line">&#123;<span class="command">count</span> : <span class="number">3</span>&#125;</span><br></pre></td></tr></table></figure></p>
<h3 id="网页分类">网页分类</h3><p>MapReduce找出哪个主题最为热门，热门与否由最近的投票决定。<br>首先，建立一个map函数，发出（emit）标签和一个基于流行度和新近程度的值。然后化简同一个标签的所有值，形成这个标签的分数。<br><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">map = <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i <span class="keyword">in</span> <span class="keyword">this</span>.tags) &#123;</span><br><span class="line"><span class="keyword">var</span> recency = <span class="number">1</span>/(<span class="keyword">new</span> <span class="built_in">Date</span>() - <span class="keyword">this</span>.date);</span><br><span class="line"><span class="keyword">var</span> score = recency * <span class="keyword">this</span>.score;</span><br><span class="line">emit(<span class="keyword">this</span>.tags[i], &#123;<span class="string">"urls"</span> : [<span class="keyword">this</span>.url], <span class="string">"score"</span> : score&#125;);</span><br><span class="line">&#125;</span><br><span class="line">&#125;;</span><br><span class="line">reduce = <span class="function"><span class="keyword">function</span>(<span class="params">key, emits</span>) </span>&#123;</span><br><span class="line"><span class="keyword">var</span> total = &#123;urls : [], score : <span class="number">0</span>&#125;</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i <span class="keyword">in</span> emits) &#123;</span><br><span class="line">emits[i].urls.forEach(<span class="function"><span class="keyword">function</span>(<span class="params">url</span>) </span>&#123;</span><br><span class="line">total.urls.push(url);</span><br><span class="line">&#125;</span><br><span class="line">total.score += emits[i].score;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">return</span> total;</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure></p>
<h3 id="MongoDB和MapReduce">MongoDB和MapReduce</h3><p>mapreduce除了map和reduce之外的其它键</p>
<ul>
<li><p>“finalize” : function</p>
<p>  将reduce的结果发送给这个键，这是处理过程的最后一步</p>
</li>
<li><p>“keeptemp” : boolean</p>
<p>  连接关闭时临时结果集合是否保存</p>
</li>
<li><p>“out” : string</p>
<p>  结果集合的名字，设定该项则隐藏含着keeptemp:true</p>
</li>
<li><p>“query” : document</p>
<p>  会在发往map函数前，先用指定条件过滤文档</p>
</li>
<li><p>“sort” : document</p>
<p>  在发往map前先给文档排序</p>
</li>
<li><p>“limit” : integer</p>
<p>  发往map函数的文档数量的上限</p>
</li>
<li><p>“scope” : document</p>
<p>  javascript代码中要用到的变量</p>
</li>
<li><p>“verbose” : boolean</p>
<p>  是否产生更加详尽的服务器日志</p>
</li>
</ul>

      
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        <ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#count"><span class="toc-number">1.</span> <span class="toc-text">count</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#distinct"><span class="toc-number">2.</span> <span class="toc-text">distinct</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#group"><span class="toc-number">3.</span> <span class="toc-text">group</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#使用完成器"><span class="toc-number">3.1.</span> <span class="toc-text">使用完成器</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#将函数作为键使用"><span class="toc-number">3.2.</span> <span class="toc-text">将函数作为键使用</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#MapReduce"><span class="toc-number">4.</span> <span class="toc-text">MapReduce</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#找出集合中的所有键"><span class="toc-number">4.1.</span> <span class="toc-text">找出集合中的所有键</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#网页分类"><span class="toc-number">4.2.</span> <span class="toc-text">网页分类</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#MongoDB和MapReduce"><span class="toc-number">4.3.</span> <span class="toc-text">MongoDB和MapReduce</span></a></li></ol></li></ol>
    
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